Hello everyone,

In the past, I've seen apologies for lengthy posts, so I'll begin by asking the questions that the following paragraphs set up. Has anyone connected an RHC-3 remote amperage control to a Dialarc 250 AC/DC (not HF), and operated the welder with the current-control switch in the non-remote position? Or the opposite situation, with no remote connected but the switch in the remote position? If so, what happened? For those who would like to read on, the following is original long-winded speculation that set up those questions. While I'm asking, for those much more familiar with the operation of a Dialarc that I'll ever be, am I even thinking along the right lines? Thanks in advance.

I've had a Dialarc 250 for some time now, and recently found an RHC-3 remote amperage control for a good price. I had hoped to find a complete RF-10 (041 111) remote current control kit, but had no luck in doing so. I tried an authorized dealer (Airgas), Miller4Less, and a company recommended by the dealer called Vintage Parts, but no one had it in stock. Nonetheless, I was able to find the RC1 (039 607) receptacle and the S3 (011 609) switch on eBay, and the other parts seemed fairly standard. I'm still looking for the label (012 305), but that's a topic for another post if I can't find or make one.

I'm not really a stranger to electricity or electronics, so it wasn't a problem to crimp some terminals on a few wires to make the leads. The instructions, form F096 982A, were straightforward, with one notable exception--had I connected the switch as directed, its baton would point toward the remote socket when in standard mode, and toward the welder's amperage dial when in remote mode. This seemed at odds not only with ergonomics, but also with an illustration of the labeling given in the instructions. Thus, I reversed the positions of leads 36 and 37. I've hooked up the remote, and everything works as expected. However, there were two possible--albeit incorrect--configurations that I didn't want to try without asking someone first.

The Dialarc apparently uses a magnetic amplifier to control the welding current. I did a little reading about them, and it appears that in this particular application, the magnetic amplifier is used essentially as a variable inductor. Apparently, the output current is limited (before rectification) by an iron-core inductor, which also has a separate winding for a DC control current. As the DC voltage, and consequently, current, in the control winding is increased, the core becomes more magnetically saturated, and thus is less able to oppose the current flow in the AC winding. In short, as current in the control winding increases, it "takes the brakes off" of the AC winding more, resulting in an increase in AC current. Since the controlling current is typically much smaller than the current that is controlled, the whole device is considered to be an amplifier.

I don't know about copyright restrictions, so I've used links rather than embedded illustrations. If this makes what I'm about to ask more difficult to follow, I apologize. On page 26 of this .pdf is a schematic of the Dialarc, including a schematic of the optional remote in a dashed box at the upper right. (For convenience I'll repeat the link to the instructions for the remote kit here.) If I'm reading these right, it appears that when the switch is in the remote position--and the remote is connected to the socket--the resistance of the remote potentiometer is inserted in series with the wiper of the welder's potentiometer. If the switch is in the standard position--and the remote is not connected to the socket--then the original configuration is effectively restored, differing only by the inserted closed switch (and resulting change in lead number assignment).

When the remote is connected--but the switch is in the standard position--the remote wiper is removed from the circuit, but the ends of its potentiometer appear to remain connected essentially in parallel to the control coil (between leads 35 and 36). When the remote is disconnected--but the switch is in the remote position--the circuit containing the control coil is open. Either of these situations could easily crop up by mistake. The latter, a disconnected remote with the switch in the remote position, should result in essentially the lowest possible amperage at the output. This seems safe enough, even if inconvenient.

It's the former, with a connected remote and the switch in the standard position, that I'm not too sure about. I measured the resistance across the remote potentiometer to be about 16 ohms, but I don't know that of the control coil. If the resistance of the control coil is considerably lower, the overall reduction from the parallel connection would be small, and little current would be shunted through the remote potentiometer. If the control coil resistance is higher, then I can see two potential problems. One is that the increase in current might exceed the rating of the circuit breaker CB1, tripping it. Otherwise--and I realize that this speculation runs counter to the theoretical operation of a parallel circuit--could the current shunted through the remote potentiometer somehow lower that in the control coil, with a corresponding reduction in voltage?

It is my hope that either mistake would result in a reduction in welding current, which I would think is the safest possibility. Given the legal climate these days, I would expect Miller to at least include some stern warning labels if mistaken operation could result in any sort of dangerous situation, even if this were only an increase in welding current.